The present invention relates to a liquid level sensor and a method of manufacturing the same. More particularly, the invention relates to a non-contact type liquid level sensor which is improved in the vibration proof performance, and reduced in a sensor output variation caused by a change of an ambient environment temperature, whereby the liquid level sensor accurately detects a liquid level, and which is well adaptable for a vehicle fuel tank or the like, and a method of manufacturing the non-contact type liquid level sensor.
Recently, a liquid level sensor of the non-contact type, for example, is mounted on a fuel tank of a vehicle, such as an automobile, and detects an amount of fuel liquid contained in the fuel tank. In a related liquid level sensor, an annular magnet which rotates with movement of a float is disposed within a frame. A hall element as a magnetoelectric transducing element is disposed at the center of the magnet, while being flush with the annular magnet. The hall element detects a change of magnetic flux developed by the turning magnet, and converts it into an electric signal. The liquid level sensor thus constructed detects a liquid level of the fuel in the fuel tank by using the electric signal. For example, the related liquid level sensor is disclosed in JP-A-2002-206959 (pp. 4 to 5, FIG. 1).
A magnet holder to which the magnet is fixed is rotatably held with a holding member formed on a surface of the frame. A pair of cores are disposed facing the magnet. A hall element is located in a gap between the cores. The cores and the hall element are disposed within the frame. The liquid level sensor thus constructed detects a change of magnetic flux caused by the turning of the magnet is detected. For example, this configuration is disclosed in JP-A-2002-206945 (pp. 3 to 4, FIG. 1).
Non-contact type liquid level sensors disclosed in the above Patent Documents follow. The hall element and the magnet are disposed in the same plane (JP-A-2002-206959). The magnet holder is rotatably held with a holding member formed on the frame surface (JP-A-2002-206945). Those technical features bring about the reduction of the thickness of the non-contact type liquid level sensor and the cost to manufacture. Neither of those patent documents, however, discloses a technique to avoid such an unwanted situation that the electronic parts come in contact with fuel, such as gasoline, and the performances of the electronic parts are deteriorated, by liquid-tightly sealing electronic parts, such as the hall element.
To maintain the high performances of the non-contact type liquid level sensor for a long period of time, some improvement is needed for the liquid level sensor. To this end, recently, a method is employed in which the electronic parts are liquid tightly sealed by welding the cover to the housing by laser welding, vibration welding or the like.
This technical method will be described below. As shown in FIG. 14, in a related non-contact type liquid level sensor 1, a housing 2 made of synthetic resin is fixedly disposed in a vehicle fuel tank 3. A fuel pump 19 is disposed within the vehicle fuel tank 3. A filter 7 is located at a suction port of the fuel pump 19. The filter filters out foreign materials contained in fuel 17. The filtered fuel is supplied to an engine or the like.
A rotary shaft 4 is rotatably disposed in a magnet containing portion 2a formed in the housing 2. A sintered magnet 5 is fit onto the outer peripheral surface of the rotary shaft 4. The sintered magnet 5 is fastened to the rotary shaft 4 by suitable means, such as bonding or engaging. The magnet 5 may be a ferrite magnet as formed by molding magnetic powder into an annular body, and radially magnetizing the resultant to have two magnetic poles.
A cover 14 made of synthetic resin is fixed to an opening part of the magnet containing portion 2a. A first end of the rotary shaft 4 is inserted into a support hole 14a formed in the cover 14, and rotatably supported thereby.
A first end of a float arm 6 is fixed to a float 8, and a second end thereof is fit into the hole of the rotary shaft 4 and fixed thereby. The float 8 vertically moves with a change of liquid level 15 of fuel in the vehicle fuel tank 3. At this time, the vertical movement is transmitted through the float arm 6 to the rotary shaft 4, which in turn rotates.
A couple of semicircular stators 9 are disposed facing an outer peripheral surface of the sintered magnet 5, while being combined into a circular configuration. The stators 9 are fixed to the housing 2 by suitable fixing member. Example of the fixing member is to insert mold the stators into the housing, to bond the stators to the housing, and to fit pins that are one-piece molded to the housing 2, into the positioning holes formed in the stators 9 and to fasten the pins by heat caulking.
A gap is present between the first ends of the coupled stators 9, and another gap is present between the second ends of the same. A phase difference of 180° is present between those gaps. A magnetoelectric transducing element 11, such as a hall element or a hall IC, is placed in one of the gaps, while being put between the couple of stators. Terminals 11a of the magnetoelectric transducing element 11 are electrically connected to a terminal 13.
When the float 8 vertically moves with a change of liquid level 15 of fuel in the vehicle fuel tank, the rotary shaft 4 rotates together with the sintered magnet 5. With rotation of the sintered magnet 5, a magnetic flux passing through the magnetoelectric transducing element 11 changes. The magnetoelectric transducing element 11 detects a change of the magnetic flux and coverts it into an electrical signal, and outputs it to the terminal 13.
As shown in FIG. 14, the terminal 13 transmits an electrical signal derived from the magnetoelectric transducing element 11 to the outside of the non-contact type liquid level sensor 1. The terminal is a plate like member which is made of conductive metal and longitudinally elongated, and a first end 13a of which is bent at a right angle to be L shaped. The terminal 13 is disposed such that it passes from the inside of a wall 2b of the housing 2 to the outside thereof, and its mid part is embedded in the wall 2b. 
A plurality of grooves 13b are formed in a surface of the mid part of the terminal 13. To embed the mid part of the terminal 13 in the wall 2b of the housing 2, the mid part of the terminal including the grooves 13b is coated with a seal coating material 18, and then the terminal 13 is insert molded into the housing 2. As a result, the seal coating material 18 is brought into close contact with the wall 2b of the housing 2, thereby securing a seal between the terminal 13 and the housing 2.
A sensor chamber 2c is liquid tightly sealed in a manner that a cover 35 made of synthetic resin is welded thereto by laser welding or the like. A plurality of terminals 13 are disposed within the sensor chamber. Electronic parts, such as the magnetoelectric transducing element 11, resistors, capacitors, both not shown, are soldered to the first ends 13a of those terminals 13. Those electronic parts are electrically arranged to form a part of the detecting circuit. Soldering portions of the magnetoelectric transducing element 11, resistors, capacitors and the like are coated with potting agent 10 after those are soldered. The coating lessens forces acting on the soldering portions which are caused by vibrations generated when the vehicle runs, thereby protecting the soldering portions against the vibrations.
As described above, the related sealing method liquid tightly seals the sensor chamber 2c by welding the cover 35 to the housing 2 by laser welding, vibration welding or the like. The sealing of the sensor chamber sealed by the related sealing method is damaged by dimensional variation and deformation of the housing 2, the cover 35 or partial poor welding. As a result, such fuel as gasoline enters the sensor chamber 2c to possibly degrade the performances of the electronic parts, such as the magnetoelectric transducing element 11, resistors and capacitors.
A laser welding machine used for welding the cover 35 to the housing 2 is expensive, and large facility investment is needed. In this respect, use of such a sealing technique has economical problems to be solved.
Further, the work to apply the potting agent to the soldering portion, and the work to apply the seal coating material to the terminal 13 before the insert molding process are inevitably manual, and troublesome. In this respective, the productivity in manufacturing the sensors is inferior.
When the housing 2 is thermally expanded or shrank, the stators 9 fixed to the housing 2 by the insert molding, bonding, or heat caulking of the pins, as shown in FIGS. 15A, 15B and 15C, are possibly angularly moved in a direction in which a radius curvature of the inner periphery of the combined stators 9 increases (direction of an arrow A) or decreases (direction of an arrow B) (see FIG. 15A), in a direction (direction of an arrow C, see FIG. 15B) in which the stators 9 separate from the sintered magnet 5, in a direction (direction of an arrow D, see FIG. 15C) in which the couple of stators 9 separate from each other, and in other directions. If the gap G between the couple of stators 9 and the gap between the stators 9 and the sintered magnet 5 are changed by the angular movement of the stators 9, then an output signal level of the magnetoelectric transducing element 11 changes to thereby adversely affect a detection precision.
The stators 9 and the terminal 13 are fixed to different members; the former is fixed the housing 2, and the latter is fixed to the terminal 13. For this reason, relative positions of the stators 9 and the magnetoelectric transducing element 11 sometimes change in accordance with change of ambient environmental temperature in which the liquid level sensor is use, vibration and the like of the non-contact type liquid level sensor 1. Owing to this, a detect output signal of the magnetoelectric transducing element 11 sometimes changes.